1. Field of the Invention
The present invention relates to a voltage controlled oscillator and, more particularly, to a voltage controlled oscillator used in a wireless communication system and having a positive modulation function.
2. Description of the Related Art
FIG. 2 shows a conventional voltage controlled oscillator having a positive modulation function. A voltage controlled oscillator 1 shown in FIG. 2 is formed by connecting an oscillation circuit 2, a resonance circuit 3, and a variable capacitance circuit 4. The oscillation circuit 2 has a power supply terminal 5 and an output terminal 6, and the variable capacitance circuit 4 has a control voltage terminal 7 and a modulation signal input terminal 8.
The variable capacitance circuit 4 is formed of two varactor diodes 9 and 10, three capacitors 11, 12, and 13, two resistors 14 and 15, and a microstrip line 16.
In the variable capacitance circuit 4, the anode of the varactor diode 9 is grounded while the cathode of the varactor diode 9 is connected to the resonance circuit 3 by the capacitor 11 and is also connected to the control voltage terminal 7 by the microstrip line 16. The control voltage terminal 7 is grounded via the capacitor 13.
The anode of the varactor diode 10 is grounded while the cathode of the varactor diode 10 is connected to the cathode of the varactor diode 9 by the coupling capacitor 12 and is also connected to the modulation signal input terminal 8 by the resistor 14. The modulation signal input terminal 8 is grounded via the resistor 15.
The oscillation circuit 2 and the resonance circuit 3 are arranged in a conventional manner and the configuration of these circuits will not be described.
In the voltage controlled oscillator 1 arranged as described above, the oscillation circuit 2 oscillates at the resonant frequency of the resonance system formed by the resonance circuit 3 and the variable capacitance circuit 4, and outputs an oscillation signal through the output terminal 6. The resonant frequency of the resonance system can be changed by changing the capacitance of the varactor diodes 9 and 10 constituting the variable capacitance circuit 4. The capacitance of each of the varactor diodes 9 and 10 can be changed by changing the voltage across the diode.
The capacitance of the varactor diode 9 constitutes a main part of the capacitance of the variable capacitance circuit 4. The cathode of the varactor diode 9 is connected to the control voltage terminal 7 to enable the capacitance of the varactor diode 9 to be changed by a DC control voltage input through the control voltage terminal 7. For example, if the control voltage is increased, the voltage across the varactor diode 9 becomes higher, thereby reducing the capacitance of the varactor diode 9. The resonant frequency of the resonance system is thereby increased. That is, the resonant frequency of the oscillation circuit 2 is increased. Conversely, if the control voltage is reduced, the oscillation frequency becomes lower. Thus, the oscillation frequency can be changed by changing the control voltage input through the control voltage terminal. Incidentally, the circuit formed by the microstrip line 16 and the capacitor 13 is provided for the purpose of preventing the impedance of an external circuit connected to the control voltage terminal 7 from influencing the capacitance of the variable capacitance circuit 4, and is arranged to make the impedance at the control voltage terminal 7 seen from the cathode of the varactor diode 9 infinite at a frequency substantially equal to the resonant frequency.
On the other hand, the varactor diode 10 is provided for modulation and the cathode of the varactor diode 10 is connected to the modulation signal input terminal 8 by the resistor 14. The capacitance of the varactor diode 10 can also be changed by the voltage of a modulation signal input through the modulation signal input terminal 8. An AC audio signal is input through the modulation signal input terminal 8. In this case, the capacitance of the varactor diode 10 changes according to a change in the voltage of the modulation signal, thereby changing the resonant frequency of the resonance system. The direction of change in resonant frequency caused by the varactor diode 10 is the same as that caused by the varactor diode 9; the resonant frequency becomes higher if the voltage of the modulation signal is increased, and becomes lower if the voltage of the modulation signal is reduced. Thus, the oscillation signal can be frequency-modulated (positively modulated) with the modulation signal input through the modulation signal input terminal 8. The resistors 14 and 15 are used to divide the modulation signal so as to adjust the voltage of the modulation signal applied to the cathode of the varactor diode 10, thereby adjusting the depth of modulation.
In the above-described variable capacitance circuit 4, the modulation signal input through the modulation signal input terminal 8 is also connected to the varactor diode 9 via the coupling capacitor 12. However, the impedance of the coupling capacitor 12 is set to such a value as to be low at the resonant frequency (ordinarily 1 GHz in portable telephones) and high with respect to the modulation signal (having frequencies about 1 kHz, since the modulation signal is an audio signal). Therefore, the modulation signal does not influence the varactor diode 9.
In the above-described conventional system, however, regarding only the variable capacitance circuit 4, the change in capacitance of the varactor diode 9 according to the control voltage input through the control voltage terminal 7 and the change in capacitance of the varactor diode 10 according to the modulation signal input through the modulation signal input terminal 8 are completely independent of each other. Therefore, the rate of change in the capacitance of the varactor diode 10 with respect to the modulation signal does not vary even when the capacitance of the varactor diode 9, which comprises the main part of the capacitance of the variable capacitance circuit 4, is changed by the control voltage input through the control voltage terminal 7. Consequently, the relative change in the capacitance of the variable capacitance circuit 4 according to the modulation signal is smaller when the capacitance of the varactor diode 9 is large, and is larger when the capacitance of the varactor diode 9 is small.
A problem in such a situation is that the rate of change in the oscillation frequency with a certain change in the modulation signal varies between the case where the oscillation frequency is high and the case where the oscillation frequency is low. In other words, the oscillation frequency-modulation sensitivity variation (modulation variation) is large. There is accordingly a need for means for correcting such a modulation variation on the side of a system using the voltage controlled oscillator, or the like. At the same time, a problem of an increased cost of use of the voltage controlled oscillator must be addressed.